The present invention relates to a micro-optic device including a mirror and a complicate structure fabricated by a deep dry etching technique such as a micro-optic device which is capable of light path switching and light intensity adjustment utilizing a mirror, and to a method of manufacturing such a device.
An optical switch has been proposed in which a technique such as a semiconductor anisotropic dry etching is used to form elements including a mirror, a hinge, an actuator and a light guide on a substrate and which has the function of switching a light path by insertion into and extraction from the light guide of the mirror.
To give a specific example, the structure of an MEMS (Micro-Electro-Mechanical System) optical switch disclosed in U.S. Pat. No. 6,315,462, issued Nov. 13, 2001, is shown in FIG. 1.
Formed in a sheet-like substrate 111 are four fiber channels 112a to 112d in a crisscross configuration. One of four areas which is defined by the fiber channels 112a and 112b represents a drive formation 111′. A slot 113 which forms an angle of 45° with each of the fiber channels 112a and 112b is formed in the drive formation 111′, and a movable rod 114 is disposed in the slot 113.
At its one end, the movable rod 114 carries a mirror 115, which is positioned at an area of intersection 116 between the fiber channels 112a to 112d. Support beams 117a and 117b have their one end connected to opposites sides of the movable rod 114 intermediate the length thereof, and these support beams 117a and 117b have their other end secured to fixed supports 119a and 119b, respectively, through leaf spring hinges 118a and 118b. In a similar manner, support beams 117c and 117d have their one end connected to opposite sides of the movable rod 114 at the other end thereof. These support beams 117c and 117d have their other end secured to the fixed supports 119a and 119b, respectively, through leaf spring hinges 118c and 118d. In this manner, the movable rod 114 is supported to be movable in the lengthwise direction. It is to be noted that the leaf springs 118a, 118b, 118c and 118d are folded back upon themselves to increase the spring length.
The movable rod 114 is driven by a comb tooth type electrostatic actuator. Specifically, movable comb tooth electrodes 121a to 121d are fixedly mounted as arrays on the support beams 117a to 117d, respectively, and mate with fixed comb tooth electrodes 122a to 122d, respectively, which are fixedly mounted on the drive formation 111′. When a voltage is applied across the movable comb tooth electrodes 121a and 121b and the fixed comb tooth electrodes 122a and 122b, an electrostatic force of attraction is developed to move the movable rod 114 in a direction toward the center of the area of intersection 116. On the other hand, when a voltage is applied across the movable comb tooth electrodes 121c and 121d and the fixed comb tooth electrodes 122c and 122d, an electrostatic force of attraction is developed to move the movable rod 114 in a direction away from the center of the area of intersection 116. By driving the movable rod 114 with the comb tooth type electrostatic actuator, it is possible to insert the mirror 115 into or to extract it from the center of the area of intersection 116.
Optical fibers 123a to 123d are respectively disposed in the four fiber channels 112a to 112d. When the mirror 115 is inserted into the center of the area of intersection 116, light which is emitted from the optical fiber 123a, for example, is reflected by the mirror 115 to impinge on the optical fiber 123d, and light which is emitted from the optical fiber 123b is reflected by the mirror 115 to impinge on the optical fiber 123c. On the contrary, when the mirror is extracted from the center of the area of intersection 116, light emitted from the optical fiber 123a impinges on the optical fiber 123c, and light emitted from the optical fiber 123b impinges on the optical fiber 123d. A switching of the light path takes place in this manner.
The micro-optic switch is manufactured by the manufacturing method shown in FIG. 2. Specifically, as shown in FIG. 2A, an SOI (Silicon On Insulator) substrate 130 of a three layer construction including a single crystal silicon substrate 131, on which an insulating layer 132 formed by a silicon oxide film is formed, and a single crystal silicon layer 133 is disposed on top of the insulating layer 132 is provided. A required mask 134 is formed on the single crystal silicon layer 133 by patterning a layer of mask material. Portions of the single crystal silicon layer 133 which are exposed through the mask 134 are subject to a deep anisotropic reactive ion etching (DRIE: Deep Anisotropic Reactive Ion Etching) to remove the single crystal silicon layer 133 until the insulating layer 132 becomes exposed, as illustrated in FIG. 2B.
A narrow width portion 135 of the single crystal silicon layer 133 as viewed in FIG. 2B represents movable parts such as the movable rod 114, the support beams 117a to 117d and leaf spring hinges 118a to 118d shown in FIG. 1 while a wide width portion 136 represents a structural body such as the fixed supports 119a and 119b shown in FIG. 1 which are fixedly mounted. FIG. 2 is an exemplary illustration of these parts.
Referring to FIG. 2B, a wet etching is applied to the exposed insulating layer 132 until a portion of the insulating layer 132 which is disposed beneath the narrow width portion 135 is removed by a side etching. As a consequence, the narrow width portion 135 will be located above the single crystal silicon substrate 131 through an air gap 137, as shown in FIG. 2C. Thus, the movable part which is formed by the narrow width portion 135 as the insulating layer 132 is removed is spaced from the single crystal silicon substrate 131 and becomes movable. It should be understood that the mirror 115 is fabricated during the etching treatment of the single crystal silicon layer 133 together with the movable rod 114, the support beams 117a to 117d and the movable comb tooth electrodes 121a to 121d. Subsequent to the wet etching operation, reflective films are formed by evaporation on the lateral wall surfaces of the mirror 115, thus completing the mirror 115.
In this manner, when the anisotropic reactive ion dry etching process is utilized, a vertical etched sidewall can be formed without being influenced by the crystalline orientation of the single crystal silicon substrate 131, thus enabling a minute structure of a complicate configuration as shown in FIG. 1 to be manufactured. While a deep etching can be achieved by a wet etching which uses an etchant solution applied to the single crystal silicon layer, it is to be noted that this wet etching exhibits an anisotropic behaviour with respect to the crystalline orientation of the silicon, and therefore it is difficult to manufacture an optical device for a micro-electromechanical system having a complicate construction as illustrated by the optical switch shown in FIG. 1. For this reason, a micro-optic device of this kind has been manufactured utilizing a deep anisotropic dry etching which utilizes a reactive ion.
However, when a deep anisotropic reactive ion etching is applied to the single crystal silicon substrate 131 so that a deep vertical etched sidewall surface can be obtained, there results an unevenness which is in excess of the order of 100 nm on the etched sidewall surface. If an etched sidewall surface having such an unevenness is used as a mirror surface for the movable mirror 115, the mirror cannot have a favorable reflection response. According to a technology disclosed in the patent literature: International Laid-Open Number WO 01/01 1411, Internationally Laid Open Feb. 15, 2001, prior to the deep anisotropic reactive ion etching, sacrificial raised layer masks 134b are formed close to and on the opposite side of a mask 134a on a portion 115a of the single crystal silicon layer 133 where the mirror 115 is subsequently to be formed in a manner completely separate from masks 134c which are associated with wide width portions 136. When the deep anisotropic reactive ion etching takes place subsequently, sacrificial raised layers 138 are formed on the opposite sides of the masked portion 115a in closely adjacent and parallel relationship threrwith. As shown in FIG. 3B, the single crystal silicon layer 133 is immersed into an etchant 139, whereby movable parts inclusive of the mirror portion 115a are free to move relative to the single crystal silicon substrate 131. At this time, the sacrificial raised layers 138 are removed without being connected to any fixing part. By choosing such a technology, the both sidewall surfaces of the mirror portion 115a can be made to be more smooth surfaces as compared with the surfaces which are obtained without forming the sacrificial raised surfaces 138, with an unevenness on the order of 30 nm or less.
However, it will be noted that there are a number of closely spaced parts such as individual comb teeth of the comb tooth electrodes 121a to 121d and 122a to 122d and folded back portions of the leafed spring hinges 118a to 118d which are closely spaced from each other. In particular, the insulating layer 132 has a thickness which is normally on the order of 3 μm at most, and the air gap between these movable parts and the single crystal silicon substrate 131 is very narrow. If fragments of the sacrificial raised layers 138 which are separated from the substrate are jammed into these narrow spaces, the movable parts may become inoperable or there results adverse influences upon the characteristic of the micro-optic device, leading to a degraded yield.
It is known that the rough silicon surfaces on the opposite sidewall surfaces of the mirror portion 115a which are formed by the deep anisotropic reactive ion dry etching may be thermally oxidized to form an oxide film of a thickness which is large enough compared with the small unevenness of the rough silicon surface, and the oxide film may be etched with a fluoric acid (HF) to provide a mirror surface of a reduced roughness (see non-patent literature: W. H. Juan and S. W. Pang, “Controlling sidewall smoothness for micromachined Si mirrors and lenses”, J. Vac. Sci. Technol. B14(6), November/December 1996, pp. 4080-4084).
However, with this technique, a time interval required to form an oxide film which is sufficiently thick with respect to the minute unevenness of the surface by the thermal oxidation is as long as ten hours, for example, requiring an increased manufacturing time for a micro-optic device, resulting in a high cost of the optical device.
This problem is not limited to the micro-optic switch, but a similar problem occurs when a micro-optic device including a mirror and a complicate structure other than the mirror is subject to a gas reactive, anisotropic dry etching to effect a deep etching reaction perpendicular to the surface of the substrate.